1. Field of the Invention
The present invention relates to an optical fiber delivery system for delivering ultrashort optical pulses and an optical system including the same.
This application claims priority from Japanese Patent Application No. 2007-112042) filed on Apr. 20, 2007, the content of which is incorporated herein by reference.
2. Description of the Related Art
In recent years, sub-picosecond ultrashort optical pulses, having high peak power and broad spectrum, have been used in various fields, such as biology, medicine, medical care, processing, and measurement. In particular, in the fields of biology and medicine, an optical pulse source that generates ultrashort optical pulses, such as a titanium: sapphire laser and a fiber laser, has been frequently used in a microscope using nonlinear optical effects, such as a multi-photon fluorescence microscope, a harmonic generation microscope, and a coherent anti-Stokes Raman scattering (CARS) microscope; a gene transfer apparatus using optical stress waves; a diffuse optical tomography apparatus and the like.
High-peak power, ultrashort optical pulses emitted from any of such optical pulse sources are transmitted to an optical apparatus, such as any of the microscopes described above, by using a reflective mirror or an optical fiber. From the viewpoints of handling and stability, it is strongly desirable to use an optical fiber to transmit ultrashort optical pulses.
However, the intense ultrashort optical pulses are broadened during the propagation in the optical fiber due to the group-velocity dispersion (GVD) effect, the nonlinear optical effect, such as a self-phase modulation (SPM) effect, and the interaction between them. Such pulse broadening is problematic in many applications.
For example, a nonlinear optical microscope, such as a multi-photon fluorescence microscope, requires high-peak power, ultrashort optical pulses. If the pulse width broadens in an optical fiber) the peak power of each of the optical pulses decreases accordingly, and hence the brightness of a fluorescence obtained by the microscope decreases.
In a multi-photon fluorescence microscope the multi-photon fluorescence intensity In and the peak power of optical pulse P0 are expressed by the following formulae (1) and (2), respectively.
                              I          n                =                              C            0                    ⁢                      P            0            n                    ⁢                      T            0                    ⁢                      f            rep                                              (        1        )                                          P          0                =                              C            1                    ⁢                                    P              av                                                      f                rep                            ⁢                              T                0                                                                        (        2        )            
In the above formulae (1) and (2), reference character n represents a natural number, which is 2, 3, and k for two-photon fluorescence, three-photon fluorescence, and k-photon fluorescence, respectively. Reference characters C0 and C1 represent constants. Reference character T0 represents the pulse width of the optical pulse. Reference character frep represents the repetition rate of the optical pulses. Reference character Pav represents the average power of the optical pulses. By using the formula (2) to rewrite the formula (1), the multi-photon fluorescence intensity Iin is expressed by the following formula (3).
                              I          n                =                  C          ⁢                                    P              av              n                                                      (                                                      f                    rep                                    ⁢                                      T                    0                                                  )                                            n                -                1                                                                        (        3        )            where C represents a constant.
The formula (3) shows that the multi-photon fluorescence intensity In decreases as the optical pulse width T0 broadens, while In increases as T0 narrows.
There is a report relating to a fiber delivery system of the ultrashort optical pulse which avoids the temporal broadening of the optical pulse [Non-patent Document 1]. The system consists of two optical fibers and a prism pair which is located between the two fibers. The pulse distortion due to the interaction between the GVD and SPM effect can be compensated in this system, and the optical pulse can have the same output temporal width with the input one.    [Non-patent Document 1] S. W. Clark et al., “Fiber delivery of femtosecond pulses from a Ti: sapphire laser,” Opt. Lett. 26, 1320 (2001).
Since the system, disclosed in Non-patent Document 1, delivers the optical pulse having the same temporal pulse width between at the input and at the output ends, it makes it possible to efficiently deliver the high-peak power ultrashort optical pulse from the optical pulse source.
However, the optics system, which is included in an optical apparatus such as a microscope using an ultrashort optical pulse, has a positive GVD value, because the optics system employs various kinds of optical elements such as lenses. Therefore, when high-peak power, ultrashort optical pulses are incident on the optical system through the optical fiber delivery system described above, the GVD effect of the optics system in the optical apparatus broadens the pulse width in the optics system, and hence ultrashort optical pulses, each having high peak power and a desired width, cannot be obtained at a desired position.